Two firstborn, breast-fed infants (delivery at home) were admitted to the hospital in a critical state of hypernatraemic dehydration. Case 1, a boy aged 13 days, had suffered 1220 g loss of weight since birth (31%), his serum sodium concentration was 180 mmol/l. Case 2, a girl aged 7 days, had lost 610 g since birth (18%); her serum sodium level was 159 mmol/l. In both cases poor professional support of lactation and lack of weight control had resulted in unnoticed severe malnutrition. After slow rehydration recovery was uneventful. Closer monitoring of babies' weight, e.g. twice a week, is advocated especially for breast-fed firstborns in the early weeks of life.